Conventionally, there is known a position detector in which an outer periphery of a round magnet is magnetized and Hall elements are arranged so as to face the magnet via a gap at positions 90 degrees away from each other (see, e.g., Patent Document 1).
FIG. 10 is a structural view of a magnetic encoder as a first prior art.
As shown in this figure, a cylindrical magnetic body 3 is coaxially disposed around an outer periphery of a disc-shaped permanent magnet 2 attached to a shaft 11. In other words, the magnet 2 and the magnetic body 3 face each other via a gap. In this gap, Hall sensors 47 and 48 are provided at positions 90 degrees away from each other so that the Hall sensors detect the magnetic flux density changes in the gap in accordance with the rotation of the permanent magnet 2 to thereby detect the rotational position thereof.
Furthermore, conventionally, there is also known a magnetic encoder for eliminating even-order and third-order harmonic components in which a total of 6 (six) magnetic field detecting elements are arranged circumferentially at positions 60 degrees in mechanical angle away from each other around a disc-shaped permanent magnet magnetized in one direction perpendicular to the rotation axis via a gap (see, e.g., Patent Document 2).
FIG. 11 is a structural view of a magnetic encoder as a second prior art.
In this figure, “41” to “46” denote 6 (six) magnetic field detecting elements arranged at positions 60 degrees in mechanical angle away from each other at the inside of the stator 30.
The magnetic field detecting elements 41 to 46 include a total of three pairs of magnetic field detecting elements, i.e., a pair of magnetic field detecting elements 41 and 42, a pair of magnetic field detecting elements 43 and 44, and a pair of magnetic field detecting elements 45 and 46, each pair being constituted by two magnetic field detecting elements arranged at positions 180 degrees in mechanical angle away from each other.
FIG. 12 is a block diagram of a signal processing circuit of this prior art.
In this figure, “51” to “53” respectively denote a first differential amplifier. The first differential amplifier 51 calculates the differential signal of the output signal V41 of the magnetic field detecting element 41 and the output signal V42 of the magnetic field detecting element 42. The first differential amplifier 52 calculates the differential signal of the output signal V43 of the magnetic field detecting element 43 and the output signal V44 of the magnetic field detecting element 44. The first differential amplifier 53 calculates the differential signal of the output signal V45 of the magnetic field detecting element 45 and the output signal V46 of the magnetic field detecting element 46. The first differential amplifiers 51 to 53 are configured to eliminate even-order harmonic components by differentiating the output signals of the pair of magnetic field detecting elements arranged at positions 180 degrees away from each other.
“54” and “55” respectively denote a second differential amplifier which calculates the differential signal of the first differential amplifiers 51 and 52 and that of the first differential amplifiers 52 and 53. Two differential output signals from which even-order harmonic components have been eliminated are added to thereby eliminate the remaining three-order harmonic components contained in the differential output signals. The output signals Va and Vb of the second differential amplifiers 54 and 55 have a relation of a sine wave vs. a cosine wave. The angle calculation circuit 56 performs the tan−1 (Va/Vb) calculation of both signals to calculate the rotation angle θ.    Patent Document 1: Japanese Unexamined Laid-open Patent Publication No. S58-162813    Patent Document 2: Japanese Unexamined Laid-open Patent Publication No. 2001-33277